Summary - Value class

Combine value class, methods and type safety to write a UserId class.

Summary - Value class

In this group you learned five aspects of value class. An @JvmInline value class wraps a single value with zero overhead. Type safety gives distinct types like UserId and OrderId.

A value class can contain methods; behavior accesses the wrapped value. A generic value class Box<T> wraps any type. Compared with a data class, a value class suits one field and a data class suits many. Each tool combines compile-time safety with runtime efficiency.

When to use value class?

Use a value class when you want to wrap a single value in a type-safe way: IDs, emails, units, labels. Choose a data class when multiple fields or copy/equals are needed.

A value class is a distinct type at compile time and the underlying type at runtime. This prevents mix-ups without extra memory. Methods and generic type parameters are also supported, so a wrapper carries behavior as well as a value. Picking the right tool keeps both safety and performance.

Exercise Define a value class with a method

Define @JvmInline value class UserId(val id: String) with fun display() = println("User: $id") inside. In main call UserId("U1").display().

Show example solution

@JvmInline
value class UserId(val id: String) {
    fun display() = println("User: $id")
}

fun main() {
    UserId("U1").display()
}

Exercise Use the generic Box<T>

Define @JvmInline value class Box<T>(val value: T). In main call println(Box(10).value) and println(Box("Hello").value).

Show example solution

@JvmInline
value class Box<T>(val value: T)

fun main() {
    println(Box(10).value)
    println(Box("Hello").value)
}

Test yourself

What is the main benefit of value class?
- More fields
- Automatic copy
- Inheritance
- Type safety, zero cost
Show answer

Type safety, zero cost — A value class provides type safety with zero overhead.
Match the value class concepts

@JvmInline value class

Wrapper
single val field

Required
fun display()

Method
Box<T>

Generic

Next lesson: What are coroutines?

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